(Not Applicable)
This invention relates to systems for slowing travel of vehicles and, more particularly, to cellular concrete arresting bed systems to safely decelerate an aircraft which runs off the end of a runway.
Aircraft can and do overrun the ends of runways raising the possibility of injury to passengers and destruction of or severe damage to the aircraft. Such overruns have occurred during aborted take-off or while landing, with the aircraft traveling at speeds to 80 knots. In order to minimize the hazards of overruns, the Federal Aviation Administration (FAA) generally requires a safety area of 1,000 feet in length beyond the end of the runway. Although this safety area is now an FAA standard, many runways across the country were constructed prior to its adoption and are situated such that water, roadways or other obstacles prevent economical compliance with the one thousand foot overrun requirement.
Several materials, including existing soil surfaces beyond the runway have been assessed for their ability to decelerate aircraft. Soil surfaces are very unpredictable in their arresting capability because their properties are unpredictable. For example, very dry clay can be hard and nearly impenetrable, but wet clay can cause aircraft to mire down quickly, cause the landing gear to collapse, and provide a potential for passenger and crew injury as well as greater aircraft damage.
A 1988 report addresses an investigation by the Port Authority of New York and New Jersey on the feasibility of developing a plastic foam arrester for a runway at JFK International Airport. In the report, it is stated that analyses indicated that such an arrester design is feasible and could safely stop a 100,000 pound aircraft overrunning the runway at an exit velocity up to 80 knots and an 820,000 pound aircraft overrunning at an exit velocity up to 60 knots. The report states that performance of an appropriate plastic foam arrester configuration was shown to be potentially xe2x80x9csuperior to a paved 1,000 foot overrun area, particularly when braking is not effective and reverse thrust is not available.xe2x80x9d As is well known, effectiveness of braking may be limited under wet or icy surface conditions. (University of Dayton report UDR-TR-a88-07, January 1988.)
More recently, an aircraft arresting system has been described in U.S. Pat. No. 5,193,764 to Larrett et al. In accordance with the disclosure of that patent, an aircraft arresting area is formed by adhering a plurality of stacked thin layers of rigid, friable, fire resistant phenolic foam to each other, with the lower-most layer of foam being adhered to a support surface. The stacked layers are designed so that the compressive resistance of the combined layers of rigid plastic foam is less than the force exerted by the landing gear of any aircraft of the type intended to be arrested when moving into the arresting area from a runway so that the foam is crushed when contacted by the aircraft. The preferred material is phenolic foam used with a compatible adhesive, such as a latex adhesive.
Tests of phenolic foam based arrester systems indicate that while such systems can function to bring aircraft to a stop, the use of the foam material has disadvantages. Major among the disadvantages is the fact that foam, depending upon its properties, can typically exhibit a rebound property. Thus, it was noted in phenolic foam arresting bed testing that some forward thrust was delivered to the wheels of the aircraft as it moved through the foamed material as a result of the rebound of the foam material itself.
Foamed or cellular concrete as a material for use in arresting bed systems has been suggested and undergone limited field testing in the prior art. Such testing has indicated that cellular concrete has good potential for use in arresting bed systems, based on providing many of the same advantages as phenolic foam while avoiding some of phenolic foam""s disadvantages. However, the requirements for an accurately controlled crushing strength and material uniformity throughout the arresting bed are critical and, so far as is known, the production of cellular concrete of appropriate characteristics and uniformity has not previously been achieved or described. Production of structural concrete for building purposes is an old art involving relatively simple process steps. Production of cellular concrete, while generally involving simple ingredients, is complicated by the nature and effect of aeration, mixing and hydration aspects, which must be closely specified and accurately controlled if a uniform end product, which is neither too weak nor too strong, is to be provided for present purposes. Discontinuities, including areas of weaker and stronger cellular concrete, may actually cause damage to the vehicle that is being decelerated if, for example, deceleration forces exceed wheel support structure strength. Such nonuniformity also results in an inability to accurately predict deceleration performance and total stopping distance. In one recent feasibility test utilizing commercial grade cellular concrete, an aircraft instrumented for recording of test data taxied through a bed section and load data was acquired. Even though steps had been taken to try to provide production uniformity, samples taken and aircraft load data from the test arresting bed showed significant variations between areas where the crush strength was excessively high and areas where it was excessively low. Obviously, the potential benefit of an arrester system is compromised, if the aircraft is exposed to forces that could damage or collapse the main landing gear.
A 1995 report prepared for the Federal Aviation Administration entitled xe2x80x9cPreliminary Soft Ground Arrester Design for JFK International Airportxe2x80x9d describes a proposed aircraft arrester. This report discusses the potential for use of either phenolic foam or cellular concrete. As to phenolic foam, reference is made to the disadvantage of a xe2x80x9creboundxe2x80x9d characteristic resulting in return of some energy following compression. As to cellular concrete, termed xe2x80x9cfoamcretexe2x80x9d, it is noted that xe2x80x9ca constant density (strength parameter) of foamcrete is difficult to maintainxe2x80x9d in production. It is indicated that foamcrete appears to be a good candidate for arrester construction, if it can be produced in large quantities with constant density and compressive strengths. Flat plate testing is illustrated and uniform compressive strength values of 60 and 80 psi over a five to eighty percent deformation range are described as objectives based on the level of information then available in the art. The report thus indicates the unavailability of both existing materials having acceptable characteristics and methods for production of such material, and suggests on a somewhat hypothetical basis possible characteristics and testing of such materials should they become available.
Thus, while arresting bed systems have been considered and some actual testing of various materials therefor has been explored, practical production and implementation of an arresting bed system which, within specified distances, will safely decelerate aircraft of known size and weight moving at a projected rate of speed off of a runway, has not been achieved. The particular material to be used, as well as the configuration and fabrication of an arresting bed, are all critical to the provision of an effective arresting bed system. To provide an effective arresting bed for vehicles of a range of sizes, weights and bed entry speeds, requires use of bed designs, materials and fabrication techniques capable of providing predictable drag effects and rates of vehicle deceleration. Computer program models or other techniques may be employed to develop drag or deceleration objectives for arresting beds, based on calculated forces and energy absorption for aircraft of particular size and weight, in view of corresponding landing gear strength specifications for such aircraft. However, such objectives remain merely an abstract goal in the absence of effective bed configurations, materials and fabrication techniques appropriate to convert arresting bed objectives into reality to achieve the desired results. As a result, prior information as to potential arresting bed materials and deceleration objectives has been inadequate to enable fabrication of a practical arresting bed suitable for use by commercial passenger aircraft and other vehicles.
Objects of the invention are, therefore, to provide new and improved vehicle arresting bed systems and such systems having one or more of the following advantages and capabilities:
assembly from pre-cast cellular concrete which has been acceptance tested;
block or aggregate assembly enabling progressive variation of both depth and compressive strength characteristics;
predetermined arresting characteristics, substantially independent of weather conditions;
long-life weather resistant construction;
ability of crash/fire/rescue vehicles to fully maneuver on an arresting bed;
ease of exit by passengers from a vehicle which has entered an arresting bed; and
ease of repair by block or aggregate replacement following use by an overrunning vehicle.
In accordance with the invention, a vehicle arresting bed system includes an initial section including first and second lateral rows of blocks of cellular concrete having a first dry density in the lower portion of a range of 12 to 22 pcf. Blocks of the second row have a height incrementally greater than the height of blocks of the first row. Also, the blocks of the first and second rows have a first compressive gradient strength to provide vehicle deceleration.
The bed system has a further section including third and fourth lateral rows of blocks of cellular concrete having a second dry density greater than the first dry density. Blocks of the fourth row have a height incrementally greater than the height of blocks of the third row and the blocks of the third and fourth rows have a second compressive gradient strength, greater than the first compressive gradient strength, to provide greater vehicle deceleration.
A vehicle arresting bed system may desirably have additional characteristics, such as the following. The blocks are preferably formed of cellular concrete having a wet density in a range of 15 to 23 pcf and cured in forms of predetermined sizes. By way of example, all of the blocks in a preferred embodiment are of the same length and width, but some are of different predetermined heights, with a first section of blocks having a 60/80 compressive gradient strength and a second section of blocks having an 80/100 compressive gradient strength. Each block typically has a predetermined compressive gradient strength over a depth of penetration from 10 to 60 percent of block thickness to provide gradual deceleration of a vehicle.
Bed systems in accordance with the invention may include composite blocks including portions of cellular concrete of different compressive gradient strengths, reinforced blocks including a lower layer grid of higher strength material, and edge members formed of cellular concrete blocks surrounding a bed of aggregate material or poured in place cellular concrete.